The objective of this project is to further the understanding of the microcirculation in the spleen and bone marrow and to study the regulation of the hematopoiesis in the mammalian embryo. The specific programs to be carried out are: (1) To develop a method for studying the intermediary circulation of the spleen and for measuring the relative blood flow of the "open" and "closed" circulation in the normal and pathological spleens, (2) To study the regulation of bone marrow blood flow, and (3) To study the control of hematopoiesis in the yolk sac of the cultured mouse embryo.